Junction-isolated depletion mode ferroelectric memory devices

ABSTRACT

Depletion-mode ferroelectric transistors are adapted for use as non-volatile memory cells. Various embodiments are described having a diode interposed between the bit line and a source/drain region of the transistor for added margin against read disturb. Various additional embodiments are described having an array architecture such that two memory cells sharing the same bit line also share the same program line. Using this configuration, non-selected cells are readily supplied with gate/source voltages sufficient to maintain the cells in a deactivated state during read and write operations on selected cells.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/306,592, filed Nov. 27, 2002 now U.S. Pat. No. 6,876,022 and titled“JUNCTION-ISOLATED DEPLETION MODE FERROELECTRIC MEMORY DEVICES, USES ANDOPERATION,” which is a divisional application of U.S. patent applicationSer. No. 09/652,557, filed Aug. 31, 2000 and issued Feb. 4, 2003 as U.S.Pat. No. 6,515,889, each of which is commonly assigned, and the entirecontents of which are incorporated herein by reference.

This application is further related to U.S. patent application Ser. No.09/653,074 filed Aug. 31, 2000 and issued Jul. 1, 2003 as U.S. Pat. No.6,587,365, which is commonly assigned.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to ferroelectric memory devices, andparticularly to memory cells and memory array architectures making useof junction-isolated ferroelectric depletion-mode field-effecttransistors.

BACKGROUND OF THE INVENTION

Ferroelectric materials are a class of materials that can be thought ofas having electrical properties somewhat analogous to the magneticproperties of ferromagnetic materials. A uniaxial ferromagnetic materialcan be magnetized in one of two directions, and thereafter will retain amagnetic field in that direction even after the applied magnetic fieldis removed; similarly, a ferroelectric material can be “polarized” ineither direction (by applying an electric field to it), and thereafterwill retain an electric field in that direction, even after the appliedelectric field is removed.

Ferroelectric materials have been successfully integrated intointegrated circuit processes, but this integration can have somedrawbacks. Ferroelectric materials having sufficient thermal stabilityfor integrated circuit processing often include incompatible metals thatmust be separated from a silicon substrate. Such ferroelectric materialsalso tend to be strong oxygen sources, increasing the risk ofundesirable oxidation of adjacent materials. Additionally, ferroelectricmaterials generally can only withstand a finite number of polarizationreversals before their performance degrades.

Ferroelectric memories exploit the properties of ferroelectricmaterials. These materials are useful in semiconductor memories as theyhave characteristics to provide a non-volatile memory function; after aferroelectric material has been polarized in one direction, it will holdthat polarization for an extended time without further power input. Incontrast, dynamic random access memory (DRAM) requires periodic refreshto maintain its data value, thus losing its data value upon the removalof its power source.

Since the physics of ferroelectric floating-gate memories are similar tostandard floating-gate memories (such as Flash memories), the sensingoperation is correspondingly similar. Typically, floating-gate memoriesare sensed by detecting the activation/deactivation of the selectedtransistor in response to a given gate/source voltage. Although atypical floating-gate memory's activation/deactivation state isdependent on a stored charge of its floating gate, and a ferroelectricfloating-gate memory's activation/deactivation state is dependent on apolarization of a ferroelectric layer, they both can exhibit this binarybehavior.

At the microscopic scale, the ferroelectric material can be seen to bedivided into domains. A domain is a volume within which the polarizationof the material is uniform. Each domain can have only two stablepolarization states. The magnitude of the polarization state of the bulkmaterial is a composite of the individual domain polarization states.

FIG. 1 schematically shows a typical hysteresis curve 12 for aferroelectric material. When the applied electric field E is increasedto a positive value E₁, the polarization of the material will increaseto a value P₁. When the applied positive field is subsequently removed,the polarization will fall back to a positive “remanent polarization”value P_(r). In a similar manner, when the applied electric field isincreased in the opposite direction, to a negative value −E₂, thepolarization of the material will go to a negative value −P₂. When theapplied negative field is subsequently removed, the polarization willfall back to a negative remanent polarization value −P_(r). Thus, thematerial can take either of two polarization states in the absence of anelectric field, depending on how it has been affected by the previouslyapplied field. For electrical circuit analysis, the polarization stateof a ferroelectric film can be thought of in terms of surface chargedensity, i.e., as amount of charge per unit area (usually written as“σ”). Curve 14 is an example of a minor hysteresis curve obtained whenthe same material is cycled between electrical potentials havinginsufficient magnitude to cause complete reversal of the polarization.

When an increasingly strong electric field is applied to a ferroelectricmaterial, more and more of the domains will change their state to lineup with the applied field. The electric field seen by any one domain isaffected by the polarization states of the other domains which arenearby. Consequently, a full reversal of polarization requires not onlysome threshold energy level, but also some delay as individual domainsalign. This is inconvenient for ferroelectric memories, since it limitsthe write speed of any such memory. Moreover, in memories that use adestructive read, i.e., a read operation using a voltage sufficient tocause reversal of polarity, this phenomenon is also an importantconstraint on read access time as the data must be rewritten aftersensing. This has been a problem with commercialization of ferroelectricmemories, since it is highly desirable for ferroelectric memories tohave access times approximately as fast as those for DRAM memories.

For the reasons stated above, and for other reasons stated below thatwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternate architecture and methods of operation of ferroelectricsemiconductor memory devices.

SUMMARY OF THE INVENTION

The above-mentioned problems with memory devices and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification.

Depletion-mode ferroelectric transistors are described herein for use asnon-volatile memory cells. Such memory cells find use in non-volatilememory devices as well as other electronic systems having non-volatilememory storage. Various embodiments are described having a diodeinterposed between the bit line and a source/drain region of thetransistor for added margin against read disturb, i.e., undesirablereversal of polarity. Various additional embodiments are describedhaving an array architecture such that two memory cells sharing the samebit line also share the same program line. Using this configuration,non-selected cells are readily supplied with gate/source voltagessufficient to maintain the cells in a deactivated state during read andwrite operations on selected cells while avoiding undesirable reversalof polarity.

For one embodiment, the invention provides a memory cell. The memorycell includes a ferroelectric layer formed overlying a substrate and acontrol gate formed overlying the ferroelectric layer and coupled to aword line. The memory cell further includes a first source/drain regionhaving a first conductivity type formed in the substrate and coupled toa program line, a second source/drain region having the firstconductivity type formed in the substrate, and a channel region havingthe first conductivity type formed in the substrate and interposedbetween the first and second source/drain regions. The memory cell stillfurther includes a diode interposed between the second source/drainregion and a bit line.

For another embodiment, the invention provides a memory cell. The memorycell includes a ferroelectric layer formed overlying a substrate and acontrol gate formed overlying the ferroelectric layer and coupled to aword line. The memory cell further includes a first source/drain regionhaving a first conductivity type formed in the substrate and coupled toa program line, a second source/drain region having the firstconductivity type formed in the substrate, and a channel region havingthe first conductivity type formed in the substrate and interposedbetween the first and second source/drain regions. The memory cell stillfurther includes a diode formed in the second source/drain region andinterposed between the second source/drain region and a bit line.

For a further embodiment, the invention provides a memory cell. Thememory cell includes a conductively-doped polysilicon layer having afirst conductivity type formed overlying a substrate having the firstconductivity type. The memory cell further includes a ferroelectrictransistor formed overlying the polysilicon layer. The transistor has acontrol gate, a ferroelectric layer interposed between the polysiliconlayer and the control gate, a first source/drain region having the firstconductivity type formed in the polysilicon layer, a second source/drainregion having the first conductivity type formed in the polysiliconlayer, and a channel having the first conductivity type formed in thepolysilicon layer interposed between the first and second source/drainregions. The memory cell still further includes a well region having asecond conductivity type opposite the first conductivity type. The wellregion is interposed between the second source/drain region and thesubstrate.

For a further embodiment, the invention provides a method of writing toa selected ferroelectric memory cell in an array of ferroelectric memorycells. The method includes applying a programming voltage to a firstword line coupled to a control gate of the selected memory cell, whereina gate/source voltage equal to the programming voltage is sufficient tocause a reversal of polarity of each memory cell. The method furtherincludes applying a fraction of the programming voltage to other wordlines coupled to control gates of non-selected memory cells notassociated with the first word line. The method still further includesapplying a ground potential to a first program line coupled to a firstsource/drain region of the selected memory cell and to a first bit linecoupled to a second source/drain region of the selected memory cellthrough a diode. The method still further includes applying the fractionof the programming voltage to other program lines coupled to firstsource/drain regions of non-selected memory cells not associated withthe first program line and to other bit lines coupled, through diodes,to second source/drain regions of non-selected memory cells notassociated with the first bit line.

For yet another embodiment, the invention provides a method of reading aselected ferroelectric memory cell in an array of ferroelectric memorycells. The method includes applying a first fraction of a programmingvoltage to a first word line coupled to a control gate of the selectedmemory cell, wherein a gate/source voltage equal to the programmingvoltage is sufficient to cause a reversal of polarity of each memorycell. The method further includes applying a ground potential to otherword lines coupled to control gates of non-selected memory cells notassociated with the first word line. The method still further includesapplying the first fraction of the programming voltage to a firstprogram line coupled to a first source/drain region of the selectedmemory cell and to other program lines coupled to first source/drainregions of non-selected memory cells not associated with the firstprogram line. The method still further includes applying a secondfraction of the programming voltage to a first bit line coupled to asecond source/drain region of the selected memory cell through a diodeand to other bit lines coupled, through diodes, to second source/drainregions of non-selected memory cells not associated with the first bitline.

The invention further provides apparatus, systems and methods of variousscope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of typical hysteresis curves for a ferroelectricmaterial.

FIG. 2 is a schematic of a portion of a memory device showing an arrayarchitecture in accordance with an embodiment of the invention.

FIGS. 3A–3D are cross-sectional views of memory cells at various stagesin their fabrication in accordance with one embodiment of the invention.

FIG. 4 is a diagram of current/voltage curves (I_(DS) VS. V_(GS)) fortwo different polarization states of a transistor in accordance with theinvention in relation to a comparable transistor without a ferroelectriclayer.

FIG. 5 is a cross-sectional view of memory cells in accordance withanother embodiment of the invention.

FIG. 6 is a cross-section view of a memory cell in accordance with afurther embodiment of the invention.

FIGS. 7A–7B are voltage diagrams of the array architecture of FIG. 2during a write operation.

FIGS. 8A–8B are voltage diagrams of the array architecture of FIG. 2during a read operation.

FIGS. 9A–9C are cross-sectional views of memory cells showing appliedvoltages and depletion/accumulation effect during various stages of aread operation.

FIG. 10 is a block diagram of a memory device in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the present embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the inventions may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that process, electrical or mechanical changes may be madewithout departing from the scope of the present invention. The termswafer or substrate used in the following description include any basesemiconductor structure. Both are to be understood as includingsilicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI)technology, thin film transistor (TFF) technology, doped and undopedsemiconductors, epitaxial layers of a silicon supported by a basesemiconductor structure, as well as other semiconductor structures wellknown to one skilled in the art. Furthermore, when reference is made toa wafer or substrate in the following description, previous processsteps may have been utilized to form regions/junctions in the basesemiconductor structure, and terms wafer or substrate include theunderlying layers containing such regions/junctions. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims and equivalents thereof.

Memory cells in accordance with various embodiments of the inventioninclude a ferroelectric (FE) field-effect transistor (FET), such as ametal-ferroelectric-metal-oxide-semiconductor (MFMOS) FET. Each FE FETof the various embodiments has at least one source/drain region havingthe same conductivity type as its channel. The FE FETs include an FEdielectric material.

The fabrication of the structures of the various example embodiments aredemonstrated using silicon MOS technology. Memory cells of the exampleembodiments are fabricated on a p-type silicon substrate. However, asnoted above, other substrates may be used for integrated circuitfabrication. Furthermore, the various embodiments could similarly befabricated on a substrate having opposite conductivity, usingappropriate changes in dopants and applied voltages. Additionally,various components of the memory cells may be fabricated in an orderdifferent from the example embodiments while still producing a memorycell in accordance with the invention.

FIG. 2 shows a sample layout of a portion of a memory device using thedisclosed memory cells. In this drawing, bit lines 209 and program lines201 are both orthogonal to the word lines 202 which they overlie.Furthermore, two memory cells coupled to the same bit line 209 will alsobe coupled to the same program line 201. Using this configuration,non-selected cells are readily supplied with gate/source voltagessufficient to maintain the cells in a deactivated state during read andwrite operations on selected cells as described with reference to FIGS.7A–7B, 8A–8B and 9A–9C.

The program lines 201 are vertically separated from the word lines 202by a layer of dielectric material. Examples of dielectric materialsinclude silicon oxides, silicon nitrides and silicon oxynitrides.Furthermore, the dielectric materials may include doped silicon oxides,such as borophosphosilicate glass (BPSG). The bit lines 209 arevertically separated from the word lines 202 and program lines 201 by afurther layer of dielectric material. Word lines 202 form the gates ofthe memory cells (not shown in FIG. 2). Program line contacts (PLCT) 211are coupled to first source/drain regions of the memory cells while bitline contacts (BLCT) 219 are coupled to second source/drain regions ofthe memory cells.

FIGS. 3A–3D show a cross-section of the embodiment of FIG. 2 taken alongdotted line A–A′ at various stages of fabrication. It will beappreciated that the program lines 201 and bit lines 209 run parallel tothe section taken, with only their contact structures 211 and 219 beingseen in FIGS. 3A–3D, while word lines 202 extend normal to thecross-section. Thus, the word lines 202 are orthogonal to the programlines 201 and bit lines 209.

The gate dielectric layer 203 is formed overlying a semiconductor regionhaving a conductivity type, such as an n-well 102 formed in a p-typesubstrate 101. Formation of the n-well 102 includes formation of thesource/drain regions and channel regions of the memory cell transistors.Note that since the transistors are depletion-mode devices, the dopinglevel of the n-well 102 will be such that the channel region can bedepleted by one of the two states of the later-deposited ferroelectriclayer. For one embodiment, the substrate 101 is doped with an n-typeimpurity, such as phosphorus, to a doping level of approximately 1.0E18cm⁻³ and to a depth of approximately 800 Å, thereby forming the n-well102. For additional embodiments, the substrate 101 is doped with ann-type impurity ranging from a doping level of approximately 4.0E18 cm⁻³at a depth of approximately 300 Å to a doping level of approximately1.0E17 cm³ at a depth of approximately 1200 Å. For yet anotherembodiment, the doping level of the source/drain regions of a transistoris the same as the doping level of the channel region of the transistor.

The gate dielectric layer 203 is a non-ferroelectric dielectricmaterial, such as a silicon oxide. The silicon oxide may be formed byconventional methods, such as thermal oxidation. As an example, thesubstrate 101 may be placed in an oxygen-containing ambient atapproximately 900° C. to grow the gate dielectric layer 203. Othermethods of forming the gate dielectric layer 203 include physical vapordeposition (PVD) and chemical vapor deposition (CVD) as is known in theart of integrated circuit fabrication. For another embodiment, the gatedielectric layer 203 is silicon nitride formed by a PVD process, such asjet vapor deposition. Other dielectric materials may be used for thegate dielectric layer 203. Specific examples include silicon oxides,silicon nitrides and silicon oxynitrides.

The floating gate 204 contains a conductive material, such asconductively-doped polysilicon, metal silicide, metal or metal alloy.Polysilicon layers are generally formed by CVD. Metal silicide layersmay be formed directly through CVD, or they may be formed sequentially,such as by depositing a layer of metal on a silicon-rich layer, andreacting the layer of metal with the underlying silicon-rich layer.Metals and metal alloys are generally formed by a PVD process, such assputtering.

The floating gate 204 will generally have the gate dielectric layer 203on one side and the FE layer 206 on the other side. As such, thefloating gate 204 may require multiple layers to provide adhesion toadjoining layers and/or to provide barrier properties for theferroelectric material. For one embodiment, the floating gate 204contains a metal layer overlying a conductively-doped polysilicon layer.For a further embodiment, the metal layer contains more than one metallayer, such as a layer of platinum overlying a layer of titanium. Foranother embodiment, the metal layer contains a layer of iridiumoverlying a layer of iridium oxide (IrO₂).

The FE layer 206 is formed overlying the floating gate 204. For oneembodiment, the FE layer 206 is a metal oxide, such as strontium bismuthtantalite (SBT) or lead zirconium titanate (PZT). Other metal oxideshaving ferroelectric properties may be used for the FE layer 206. Someexamples include lanthanum-doped PZT (PLZT), lithium niobate (LiNbO₃),or additional metal oxides having a perovskite crystalline structure.The metal oxide may be formed by such CVD techniques as metal organicdecomposition. For one embodiment, the floating gate 204 is eliminatedfor cases where the gate dielectric layer 203 is compatible with the FElayer 206, such that the FE layer 206 is overlying and adjoining thegate dielectric layer 203. For a further embodiment, the gate dielectriclayer 203 and the floating gate 204 are eliminated where thesemiconductor material, e.g., n-well 102, is compatible with the FElayer 206, such that the FE layer 206 is overlying and adjoining thesemiconductor material.

A control gate 207 is formed overlying the FE layer 206. The controlgate 207 contains a conductive material. For one embodiment, the controlgate 207 includes a barrier layer, such as a metal barrier layer. For afurther embodiment, the control gate 207 contains more than one layer.As one example, the control gate 207 may contain a layer of titaniumoverlying a layer of platinum. As another example, the control gate 207may contain a metal layer overlying a conductive metal oxide layer, suchas a layer of iridium overlying a layer of iridium oxide.

A cap layer 212 is generally formed overlying the control gate 207 toact as an insulator and barrier layer for the word line stack. The caplayer 212 contains an insulator and may include such insulators assilicon oxide, silicon nitride, and silicon oxynitrides. For oneembodiment, the cap layer 212 is silicon nitride, formed by such methodsas CVD or PVD.

The gate dielectric layer 203, the floating gate 204, the FE layer 206,the control gate 207 and the cap layer 212 are subsequently patterned todefine the word line stack as depicted in FIG. 3A. Patterning caninclude use of standard photolithographic techniques. As an example, alayer of photoresist may be deposited, exposed with an energy source,and developed to expose portions of the word line stack. Material isthen removed from the exposed portions of the word line stack, includingthe exposed portions of the gate dielectric layer 203, the floating gate204, the FE layer 206, the control gate 207 and the cap layer 212. Suchremoval may typically include chemical or ion etching. The resist isthen removed, such as by plasma etch.

While the definition of the word line stack in the foregoing descriptionis performed in a single patterning step, the layers may be individuallypatterned. For one embodiment, the gate dielectric layer 203 and thefloating gate 204 are patterned prior to formation of the FE layer 206.The FE layer 206 is then patterned to have a width less than the widthof the previous layers. Such a structure can provide additionaldiffusion barrier characteristics upon formation of the control gate207; the control gate 207 can overlie both the surface and sidewalls ofthe FE layer 206.

The sidewalls of the word lines 202 are then insulated using sidewallspacers 214. The sidewall spacers 214 contain an insulator and mayinclude the same materials as the cap layer 212. The sidewall spacers214 are typically formed by blanket depositing an insulating layer, suchas a layer of silicon nitride, over the entire structure and thenanisotropically etching the insulating layer to preferentially removethe horizontal regions and the leave only the vertical regions adjacentthe sidewalls of the word line stacks. The resulting transistor is shownin FIG. 3B.

A bulk insulator layer 220 is formed overlying the word line stacks andpatterned to define contact holes 214 for the program line contacts asshown in FIG. 3C. Conductive plugs 211 are formed in the contact holes214 and the program lines 201 are formed overlying the plugs 211 and thebulk insulator layer 220 (outside the plane of the figure). Example plugstructures include a conductively-doped polysilicon plug material with ametal silicide interface between the n-well 102 and the plug material.The plugs 211 are coupled to the program lines 201 using extensions tolaterally offset the program lines 201 from their associated bit lines209 in order to facilitate subsequent formation of the bit linecontacts.

The bulk insulator layer 220 is extended in FIG. 3D and patterned todefine contact holes for the bit line contacts. Conductive plugs 219 areformed in the contact holes and the bit lines 209 are formed overlyingthe plugs 219 and the extended bulk insulator layer 220 as depicted inFIG. 3D. The conductive plugs 211 and 219 provide electricalcommunication between the semiconductor material and the program lines201 and bit lines 209, respectively.

The bit lines 209 and program lines 201 are coupled to columns of memorycells of a memory array. Each contains a conductive material. For oneembodiment, the bit line 209 and/or program line 201 contains a metal.For another embodiment, the bit line 209 and/or program line 201contains a metal alloy. For a further embodiment, the bit line 209and/or program line 201 contains more than one layer of conductivematerial. The bit lines 209 and program lines 201 may make use of aninsulative cap layer as with the word lines 202. The word lines 202further contain a conductive material. The word lines 202 may double asthe control gate 207 of the memory cell transistors and thus be coupledto and have the same construction as the control gate 207 described withreference to FIGS. 3A–3D. Alternatively, the cap layer 212 may beeliminated and the word lines 202 may be formed to overlie and couple tothe control gates 207. The word lines 202 are coupled to rows of memorycells of the memory array.

Each transistor of each word line 202 can have a first programmed staterepresenting a first data value, such as a data value of 1, or a secondprogrammed state representing a second data value, such as a data valueof 0. The programmed state is a function of the polarization of the FElayer 206. Word line 202A depicts an FE layer 206 programmed to thesecond programmed state. In the second programmed state, the additionalnegative voltage at the gate dielectric layer 203 causes a depletionlayer to form underneath the gate, so that the transistor is deactivatedat a zero gate/source voltage corresponding to an “off” state. Word line202B depicts an FE layer 206 programmed to the first programmed state.In the first programmed state, the additional positive voltage at thegate dielectric layer 203 will attract electrons, such that thetransistor is activated at a zero gate/source voltage corresponding toan “on” state.

FIG. 4 shows example current/voltage curves (I_(DS) VS. V_(GS)) for thetwo different polarization states of one embodiment of the transistor. Aferroelectric transistor that is in the first programmed state will turnon at a lower gate/source voltage V_(GS) (in this example, whereV_(GS)=V_(G2)=−2V) relative to a comparable depletion-mode transistorwithout a ferroelectric layer, shown as “A” in FIG. 4. Likewise, aferroelectric transistor that is in the second programmed state willturn on at a higher V_(GS) (in this example, where V_(GS)=V_(GI)=1V).While specific potential levels were used in the example, FIG. 4 isprovided for illustrative purposes to show that varying the polarizationof the ferroelectric layer 206 will alter the threshold voltage of thetransistor, thus determining whether the transistor will be activated ordeactivated in response to a given V_(GS). Accordingly, the invention isnot limited to the specific values of V_(GS).

In another embodiment, shown in FIG. 5, the disclosed array is seenformed on a silicon-on-insulator substrate. Complete isolation of theactive areas from the underlying silicon substrate 101 is provided by aburied oxide (BOX) layer 104 or other layer of dielectric material,while shallow trench isolation (STI) areas 105 separate adjacent pairsof transistors. This cross-section is similar to the cross-section seenin FIG. 3, differing primarily in the area of device isolation.Formation of the buried oxide layer 104 and areas of shallow trenchisolation 105 is well known in the art. Furthermore, formation of thememory cells can be accomplished as described with reference to FIG. 3.Accordingly, detailed discussion of fabrication techniques is omittedfor clarity.

The embodiment of FIG. 5 shows further that a p-well 108 can be formedbeneath the bit line contacts 219 to be interposed between the bit line209 and a source/drain region of the transistor. The p-well 108 may beformed by doping an exposed portion of the n-well 102 with a p-typeimpurity, such as boron, after patterning the bulk insulator layer todefine the contact hole for the bit line contact and before formation ofthe contact plug. Additionally, doping of the p-well 108 may occur priorto formation of the bulk insulator layer, using a separate mask. Suchdoping is usually performed through ion implantation techniques.However, other methods are known such as diffusion techniques usinggaseous, liquid or solid dopant sources.

The pn junction between the n-well 102 and the p-well 108 forms a diodeproviding isolation between the source/drain region and the bit lineduring read/write biasing for added margin against read disturb. Thisdiode configuration may also be used in the embodiment of FIGS. 3A–3D.However, the n-well 102 in FIGS. 3A–3D must be sufficiently deeper thanthe p-well 108 below the bit line contact 219 in order to avoid shortingof the p-well 108 to the underlying p-type substrate 101. The diodes ofthe various embodiments are isolated from the control gates 207 and,thus, the word lines 202.

In a further embodiment, the channel can be formed of polysilicon,rather than monocrystalline silicon. FIG. 6 is an example of a memorycell formed over polysilicon. In the embodiment shown in FIG. 6, thecells are formed so that they may overlie the sense amplifiers androw-column decode circuits (not shown in FIG. 6) formed on a substrate601.

For the embodiment depicted in FIG. 6, the substrate 601 is an n-typesubstrate. The substrate 601 could further be a p-type substrate or adoped well of a first conductivity type, such as an n-well, formed in adoped substrate of a second and opposite conductivity type, such as ap-type substrate. Formation of the memory cell follows semiconductorfabrication techniques of the type described with reference to FIGS.3A–3D, so details are omitted for clarity.

The memory cell includes a transistor as a portion of a word line 202.The transistor may have the same construction as that depicted in FIGS.3A–3D, such as the gate dielectric layer 203, floating gate 204, FElayer 206, control gate 207, cap layer 212 and sidewall spacers 214. Theword line 202 is formed overlying a conductively-doped polysilicon layer602. The polysilicon layer 602 has a first conductivity type, such as ann-type conductivity. The first source/drain region of the transistor iscoupled to a program line 201 through a conductive plug 611. The secondsource/drain region of the transistor is coupled to a bit line 209through a conductive plug 619. A well 608 having the second conductivitytype is formed in the substrate 601 interposed between the secondsource/drain region and the bit line 209, the substrate 601 having thefirst conductivity type. Thus, a diode is formed between the bit line209 and a source/drain region of the memory cell transistor.

The word lines 202 and bit lines 209 run normal to the face of FIG. 6for such an embodiment. Likewise, the program lines 201 for thisembodiment run parallel to the face of FIG. 6.

An overview of the read and write operations will now be provided, withparticular reference to the array architecture of FIGS. 2 and 3A–3D. Itis assumed for the following examples that the voltage drop needed tochange the state of the ferroelectric layer is approximately 2V. Thegate/source voltage V_(GS) generally can be broken down into two primarycomponents, i.e., a voltage drop across the gate dielectric layer and avoltage drop across the ferroelectric layer. Determination of thevoltage drop across each of these dielectric layers of the gate stackcan be made using standard calculations for the voltage drop across aseries capacitance.

The programming voltage V_(pp) must be sufficient to produce a voltagedrop across the ferroelectric layer that is equal to or greater than theremanent coercivity of the ferroelectric layer, i.e., an electric fieldsufficient to cause reversal of polarity of the ferroelectric layer. Asnoted above, this is assumed to be approximately 2V for the exampleembodiment. While the value of V_(pp) will depend upon the chosenfabrication materials and transistor dimensions, as used in the examplesherein V_(pp) will be presumed to have a magnitude sufficient to causereversal of polarity of the given ferroelectric layer when appliedacross the gate of the transistor. While it is generally preferred thatV_(pp) have the minimum magnitude necessary to cause reversal ofpolarity (while accounting for engineering margins), higher values canbe used provided the resulting voltage drops across non-selected cellsdoes not exceed the remanent coercivity of the ferroelectric layer ofany such non-selected cell. For the example embodiments, V_(pp) isapproximately 6V.

Write Operation

FIGS. 7A–7B show the voltages applied to the array for writing a firstdata value, e.g., a data value of 1, or a second data value, e.g., adata value of 0, respectively, to the cell located in the lowerleft-hand corner of each of the drawings and represented by theintersection of WL0 and BL0. In FIG. 7A, to write the first data value,the bit line, program line, and word line of all non-selected rows andcolumns (BL1, PL1, and WL1 in this drawing) are set to some fraction ofV_(pp) in order to avoid disturbing the polarity of the non-selectedmemory cells. For one embodiment, the bit line, program line, and wordline of all non-selected rows and columns are set to approximatelyV_(pp)/2 (3V in this example). The selected bit line (BL0) and programline (PL0) are set to a ground potential, i.e., 0V. The selected wordline (WL0) is set to V_(pp) (6V in this example). By applying a voltagedifferential across the ferroelectric layer equal to or exceeding theprogramming voltage, the cell can be forced to a data value of 1.Furthermore, as seen in the figure, the change in voltage (ΔV) seenacross the non-selected cells is either 0V (for a cell in which neitherthe row nor column was selected) or V_(pp)/2 (for a cell in which eitherthe row or the column, but not both, was selected), neither of which issufficient to reverse the cell's polarity. Thus, data values of thenon-selected cells are not altered during the write operation of theselected cell.

In FIG. 7B, writing the second data value is shown. Non-selected rowsand columns again have their bit lines, program lines, and word linesset to some fraction of V_(pp), such as V_(pp)/2. In the selectedcolumn, the bit line and program line are set to V_(pp), and in theselected row, the word line is set to 0V. Again, non-selected cells seea ΔV of 0V or of −V_(pp)/2, neither of which will change the state ofthese cells, but the selected cell will see a ΔV of −V_(pp), which issufficient to cause a cell having the first data value to reverse itspolarity.

In write mode, the resulting matrix of voltages seen by the cells willtherefore be as shown in Table 1 below (where two values are shown, thefirst is for writing the first data value and the second, inparenthesis, is for writing the second data value).

TABLE 1 Second First Source/ Source/Drain Drain Region Region GateV_(GS) Selected Cell 0 V (V_(pp)) 0 V (V_(pp)) V_(pp) (0 V) V_(pp)(−V_(pp)) Half-selected V_(pp)/2 V_(pp)/2 V_(pp) (0 V) V_(pp)/2(−V_(pp)/2) (same row) Half-selected 0 V (V_(pp)) 0 V (V_(pp)) V_(pp)/2V_(pp)/2 (−V_(pp)/2) (same column) Non-selected V_(pp)/2 V_(pp)/2V_(pp)/2 0 VRead Operation

FIGS. 8A–8B and 9A–9C demonstrate an example of the READ operation ofthe cell. FIG. 9A shows a cross-section of two cells, giving thevoltages at which they are normally held in stand-by mode; in this case,approximately V_(pp)/2 (3V in this example) on all lines. Before theread operation, the word line voltages are dropped to the groundpotential, as shown in FIG. 8A, from this stand-by mode. In thisexample, the resulting effective gate/source voltage V_(GS) on everycell is thus approximately −2V. As the curves of FIG. 4 show, no cellsare able to turn on at this V_(GS), so all cells are shut off. FIG. 9Bshows a cross-section of two cells during this initialization phase,demonstrating the depletion region that is formed under theseconditions.

In the read phase, shown in FIG. 8B, the selected word line WL0 isbrought up to approximately V_(pp)/3 or approximately 2V. Concurrently,each program line voltage is dropped to approximately V_(pp)/3 orapproximately 2V. This means that V_(GS) for the selected cells is nowapproximately −0.5V. As FIG. 4 shows, cells programmed to a firstprogrammed state are able to conduct at this voltage, but cellsprogrammed to the second programmed state are not. Conduction in thisexample will be from the selected bit line to its corresponding programline as the program line is at a smaller fraction of the programmingvoltage than the bit line. Suitable sensing architectures will detect acurrent drain, and thus a voltage drop, on the selected bit line. Forsensing architectures adapted to detect an incoming current to the bitline, and thus a voltage rise on the selected bit line, voltages of thebit lines and program lines would correspondingly be swapped.

During the read phase, the V_(GS) of the non-selected cells remainsbelow their turn-on point, while the V_(GS) of the selected cell issufficient to cause activation of the transistor if it is in the firstprogrammed state and insufficient to cause activation of the transistorif it is in the second programmed state. FIG. 9C shows a cross-sectionof the same two cells, where the left-hand cell is being read and isprogrammed to the first programmed state. This transistor will turn onand pull its respective bit line down. Conventional sensingarchitectures and methods can be used to sense the conducting state ofthe selected cells.

Devices and Systems

FIG. 10 shows a general block diagram of a memory device 1050incorporating ferroelectric floating-gate memory cells and arrayarchitectures in accordance with the various embodiments of theinvention. The ferroelectric memory device 1050 is coupled to aprocessor 1051 to form an electronic system. The memory device includesa memory array 1052, column decoder 1054 and row decoder 1056, and acontrol circuit 1058. The memory array 1052 contains memory cellsarranged in rows and columns. The memory array 1052 contains theferroelectric floating-gate memory cells and/or array architectures inaccordance with the various embodiments of the invention.

The memory device 1050 further includes input 1060 and output 1062buffers connected to data input and data output lines, respectively. Thedata input and output lines can be multiplexed together, but have beenillustrated separately for simplicity. Address lines 1063 are providedas input to the column decoder 1054 and row decoder 1056 to address aportion of the memory array 1052.

In operation, the memory device control circuit 1058 responds to controlinputs 1059 from the processor 1051 to control operations performed onthe memory array 1052. In particular, the control circuit 1058 is usedto read data from and write data to the memory array 1052. During one ofthese access operations, an address provided on the address lines 1063is decoded by the row decoder 1056 to activate a word line, therebyaccessing a row of the memory array 1052. Likewise, an address providedon the address lines 1063 is decoded by the column decoder 1054 toactivate at least one bit line, thereby accessing at least one column ofthe memory array 1052. An addressed memory cell is located at theintersection between each activated word line and each activated bitline. During a read operation, the data stored in the addressed memorycell(s) is then transferred to the output buffer 1062 and provided onthe data output lines. In a write operation, the addressed memory cellis accessed and data provided on the data input lines is stored in thecell.

CONCLUSION

Depletion-mode ferroelectric transistors have been described for use asnon-volatile memory cells. Such memory cells find use in non-volatilememory devices as well as other electronic systems having non-volatilememory storage. Various embodiments are described having a diodeinterposed between the bit line and a source/drain region of thetransistor for added margin against read disturb. Various additionalembodiments are described having an array architecture such that twomemory cells sharing the same bit line also share the same program line.Using this configuration, non-selected cells are readily supplied withgate/source voltages sufficient to maintain the cells in a deactivatedstate during read and write operations on selected cells.

While specific dimensions were referred to in the example embodiments,the invention is not limited to the specific dimensions provided. It isrecognized that there is a continuing drive to reduce device dimensionsin integrated circuit manufacture. Accordingly, the referenceddimensions are intended only as guidelines under current manufacturingpractices.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe invention will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the invention. It is manifestly intended that thisinvention be limited only by the following claims and equivalentsthereof.

1. A memory cell, comprising: a conductively-doped polysilicon layerhaving a first conductivity type formed overlying a substrate having thefirst conductivity type; a ferroelectric transistor formed overlying thepolysilicon layer, wherein the transistor has a control gate, aferroelectric layer interposed between the polysilicon layer and thecontrol gate, a first source/drain region having the first conductivitytype formed in the polysilicon layer, a second source/drain regionhaving the first conductivity type formed in the polysilicon layer, anda channel having the first conductivity type formed in the polysiliconlayer interposed between the first and second source/drain regions; anda well region having a second conductivity type opposite the firstconductivity type, wherein the well region is interposed between thesecond source/drain region and the substrate.
 2. The memory cell ofclaim 1, wherein the well region is formed in the substrate and iscoupled to the second source/drain region by a conductive plug.
 3. Thememory cell of claim 1, further comprising: a gate dielectric layerinterposed between the conductively-doped polysilicon layer and theferroelectric layer; and a floating gate interposed between the gatedielectric layer and the ferroelectric layer.
 4. The memory cell ofclaim 3, wherein the floating gate comprises a conductive material. 5.The memory cell of claim 4, wherein the conductive material includes atleast one material selected from the group consisting ofconductively-doped polysilicon, metal silicides, metals and metalalloys.
 6. The memory cell of claim 5, wherein the floating gatecomprises a metal layer overlying a conductively-doped polysiliconlayer.
 7. The memory cell of claim 1, wherein the ferroelectric layercomprises a material selected from the group consisting of strontiumbismuth tantalite, lead zirconium titanate, lanthanum-doped leadzirconium titanate, lithium niobate and metal oxides having a perovskitecrystalline structure.
 8. The memory cell of claim 1, furthercomprising: a gate dielectric layer interposed between theconductively-doped polysilicon layer and the ferroelectric layer;wherein the ferroelectric layer is overlying and adjoining the gatedielectric layer.
 9. The memory cell of claim 1, wherein theferroelectric layer is overlying and adjoining the conductively-dopedpolysilicon layer.
 10. The memory cell of claim 1, wherein theferroelectric transistor is formed overlying row-column decode circuitsadapted for accessing the memory cell, the row-column decode circuitsformed on the substrate.
 11. A memory cell, comprising: aconductively-doped polysilicon layer having a first conductivity typeformed overlying a substrate having the first conductivity type; aferroelectric transistor formed overlying the polysilicon layer, whereinthe transistor has a control gate coupled to a word line, aferroelectric layer interposed between the polysilicon layer and thecontrol gate, a first source/drain region having the first conductivitytype formed in the polysilicon layer and coupled to a program line, asecond source/drain region having the first conductivity type formed inthe polysilicon layer, and a channel having the first conductivity typeformed in the polysilicon layer interposed between the first and secondsource/drain regions; and a well region having a second conductivitytype opposite the first conductivity type, wherein the well region isformed in the substrate and wherein the well region is interposedbetween the second source/drain region and a bit line.
 12. The memorycell of claim 11, further comprising: a gate dielectric layer interposedbetween the conductively-doped polysilicon layer and the ferroelectriclayer; and a floating gate interposed between the gate dielectric layerand the ferroelectric layer; wherein the floating gate comprises aconductive material.
 13. The memory cell of claim 12, wherein thefloating gate comprises a metal layer overlying a conductively-dopedpolysilicon layer.
 14. The memory cell of claim 11, further comprising:a gate dielectric layer overlying and adjoining the conductively-dopedpolysilicon layer; wherein the ferroelectric layer is overlying andadjoining the gate dielectric layer.
 15. The memory cell of claim 11,wherein the ferroelectric layer is overlying and adjoining theconductively-doped polysilicon layer.
 16. The memory cell of claim 11,wherein the ferroelectric transistor is formed overlying row-columndecode circuits adapted for accessing the memory cell, the row-columndecode circuits formed on the substrate.
 17. A memory cell, comprising:a conductively-doped polysilicon layer having a first conductivity typeformed overlying a substrate having the first conductivity type; aferroelectric transistor formed overlying the polysilicon layer, whereinthe transistor has a control gate, a ferroelectric layer interposedbetween the polysilicon layer and the control gate, a conductivefloating gate interposed between the ferroelectric layer and thepolysilicon layer, a gate dielectric layer interposed between theconductive floating gate and the polysilicon layer, a first source/drainregion having the first conductivity type formed in the polysiliconlayer, a second source/drain region having the first conductivity typeformed in the polysilicon layer, and a channel having the firstconductivity type formed in the polysilicon layer interposed between thefirst and second source/drain regions; and a well region having a secondconductivity type opposite the first conductivity type, wherein the wellregion is interposed between the second source/drain region and thesubstrate and coupled to the second source/drain region by a conductiveplug.
 18. The memory cell of claim 17, wherein the conductive floatinggate includes at least one material selected from the group consistingof conductively-doped polysilicon, metal silicides, metals and metalalloys.
 19. The memory cell of claim 18, wherein the floating gatecomprises a metal layer overlying a conductively-doped polysiliconlayer.
 20. A memory cell, comprising: a conductively-doped polysiliconlayer having a first conductivity type formed overlying a substratehaving the first conductivity type; a ferroelectric transistor formedoverlying the polysilicon layer, wherein the transistor has a controlgate, a ferroelectric layer interposed between the polysilicon layer andthe control gate, a gate dielectric layer interposed between theferroelectric layer and the polysilicon layer, a first source/drainregion having the first conductivity type formed in the polysiliconlayer, a second source/drain region having the first conductivity typeformed in the polysilicon layer, and a channel having the firstconductivity type formed in the polysilicon layer interposed between thefirst and second source/drain regions; and a well region having a secondconductivity type opposite the first conductivity type, wherein the wellregion is interposed between the second source/drain region and thesubstrate and coupled to the second source/drain region by a conductiveplug.
 21. A memory cell, comprising: a conductively-doped polysiliconlayer having a first conductivity type formed overlying a substratehaving the first conductivity type; a ferroelectric transistor formedoverlying the polysilicon layer, wherein the transistor has a controlgate, a ferroelectric layer interposed between the polysilicon layer andthe control gate, a first source/drain region having the firstconductivity type formed in the polysilicon layer, a second source/drainregion having the first conductivity type formed in the polysiliconlayer, and a channel having the first conductivity type formed in thepolysilicon layer interposed between the first and second source/drainregions; and a well region having a second conductivity type oppositethe first conductivity type, wherein the well region is interposedbetween the second source/drain region and the substrate and coupled tothe second source/drain region by a conductive plug; wherein theferroelectric layer is adjoining the polysilicon layer.